Conventionally, most packaging containers for storing liquid food such as milk, juice, liquor, cola drink, or coffee are formed by folding a packaging material prepared from a laminate including paper, plastic, metallic foil, metal-deposited film, etc., in combination. Edge portions of the folded packaging material are overlapped with each other and welded. In order to prevent liquid food from leaking or oozing through the overlap portions to the outside of the packaging container, the overlap portions facing the interior of the packaging container are strongly bonded together by use of a strip tape.
The strip tape is formed of, for example, a laminate including a center layer of a polar resin and a polyolefin layer laminated on each face of the center layer. When the laminate is formed, at least one of the center layer and the polyolefin layer is subjected to anchor coating, and then the polyolefin layer is laminated on each face of the center layer. Usually, a thermosetting adhesive obtained by dissolving, for example, a polyester resin in an organic solvent is used as an anchor-coating agent for carrying out the aforementioned anchor coating.
However, when the strip tape including the center layer and the polyolefin layer, at least one of the layers being subjected to anchor coating, is used, liquid food is absorbed in the strip tape from an edge portion thereof, and thus the interlayer bonding force is lowered. When the packaging container is heated or cooled drastically during use thereof, the interlayer bonding force is further lowered. As a result, the center layer and the polyolefin layer exfoliate from each other, resulting in a deterioration in the function of the strip tape for protecting liquid food contained in the packaging container. In addition, the strip tape having the aforementioned structure has low thermal stability and poor oxygen-barrier property.
Moreover, since the aforementioned anchor-coating agent contains an organic solvent, when anchor coating is carried out or when the polyolefin layer is laminated on each face of the center layer, the organic solvent disperses, and thus the natural environment or the working environment is polluted.
In order to solve the aforementioned problems in relation to the conventional strip tape, an object of the present invention is to provide a strip tape which does not undergo lowering of bonding force and which does not cause pollution of the natural environment or the working environment.